A study on the effectiveness of cut off wall system to mitigate the ground displacement induced by tunneling in soft soil

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY Mr VU NAM CHIEN NGUYEN STUDY ON THE EFFECTIVENESS OF CUT-OFF WALL SYSTEM TO MITIGATE GROUND DISPLACEMENTS INDUCED BY TUNNELING MAJOR: INFRASTRUCTURE OF ENGINEERING CODE: 8900201.04QTD RESEARCH SUPERVISOR: Dr MINH NGAN VU Hanoi, 2021 ACKNOWLEDGEMENT I would like to express my sincere appreciation for the lecturers of Master of Infrastructure Engineering Program for their help during my study journey at Vietnam Japan University (VJU) To begin with, I very appreciate Dr Vu Minh Ngan, who had guided me to conduct this thesis for over one year He spent a lot of time telling me complicated issues in geotechnical engineering, especially his own book about tunneling is very helpful for my beginning with this aspect Not about knowledge, he also taught me valuable lesson about the seriousness and carefulness in scientific research These valuable lessons will follow me throughout the future occupation I would like to acknowledge the sincere inspiration from Prof Nguyen Dinh Duc and Prof Hironori Kato Their lectures cover not only specialist knowledge but also the responsibility and mission of a new generation of Vietnam I am grateful to Dr Phan Le Binh and Dr Nguyen Tien Dung for his support in the last two years since I have studied at Vietnam Japan University Thanks to the professors, I have learned the professional courtesy of Japanese people as well as Japanese culture I would also like to acknowledge the staff of Vietnam Japan University, Mr Bui Hoang Tan for their help and support Besides, thanks MIE 4th friends for our study journey together I believe the period will accompany us even after the defense I hope we will all be successful in the future Finally, I want to express my gratitude to my parents and my girlfriend for their unconditional support in the tough time Their support, spoken or unspoken, has helped me complete my master thesis until the end ABSTRACT The purpose of ensuring safety when designing and constructing tunnels is an issue that needs to be researched to develop Vietnam's urban railway system in near future From such reality, there are many technical challenges are set to be studied in order to ensure the safe operation of TBM excavators and structures in the affected area Cut off wall method is one of the high effective solutions in mitigating the settlement in the world With the first application in Hochiminh city in Vietnam recently, the system has been working as a separating method which protect the Opera House-one of the largest historical building The wall system was constructed by jetgrouting method and worked against the affect from TBM excavation underground by its advantage with Vietnamese soil condition This study will verify the effectiveness range of the solution, which has been implemented, by changing of characteristic parameters as thickness and elastic modulus of wall system The result of effective range produced by FEM and semi-empirical methods will contribute to the more appropriate selection of reasonable Cut-off wall structure for each particular case in Vietnam TABLE OF CONTENT CHAPTER INTRODUCTION .1 1.1 General background 1.2 Problem statement 1.3 Aims of the study .3 1.4 Objectives of the study .4 CHAPTER LITERATURE REVIEW 2.1 Background 2.1.1 Overview of tunneling research in the world 2.1.2 Vietnamese authors gradually approach the research on tunnel issues 2.2 Overview of settlements induced by tunneling 2.2.1 The principle of settlements induced by tunneling 2.2.1.1 Ground displacements surrounding the tunnel 2.2.2 Overview of ground strengthening 10 2.2.3 Ground strengthening methods in tunneling 12 2.2.3.1 Strengthening by changing soil properties methods 12 2.2.3.2 Strengthening without changing soil properties 14 CHAPTER ANALYSIS METHODOLOGY 17 3.1 Methodology 17 3.2 Mitigating measure selection 18 3.2.1 Using cut-off wall method to mitigate the settlement 19 3.2.1.1 Effect of rough wall system on ground displacement .22 3.2.1.2 Effect of smooth wall system on ground displacement .22 CHAPTER DATA ANALYSIS AND DISCUSSION 25 4.1 Location and scope 25 4.2 Analyze the settlement due to tunneling 26 4.2.1 Input parameters .26 4.2.1.1 Geological conditions 26 4.2.1.2 Design specification of TBM 28 4.2.2 Calculate settlement based on semi-empirical method 29 4.2.3 Calculate settlement based on FEM method 30 4.2.4 Settlement results 31 4.3 Research on the solution of the wall system built by Jet-grouting 32 4.3.1 Input parameters .33 4.3.2 Result of calculation .34 4.3.2.1 Relationship E-S with δ value fixed 34 4.3.2.2 Relationship δ -S with E value fixed .36 4.3.2.3 Relationship E-S of surface settlement 37 4.3.3 Verify the effectiveness with site location monitoring data 38 CHAPTER CONCLUSION & DISCUSSION 41 5.1 Conclusion 41 5.2 Discussion 41 LIST OF TABLES Table 4.1 Soil layers at borehole/ location of research 27 Table 4.2 Geological input parameters 27 Table 4.3 Design input parameter of TBM 28 Table 4.4 Design input parameter of TBM shield 29 Table 4.5 Design input parameter of cut-off wall system 33 Table 4.6 Monitoring settlement data at reinforced locations 39 i TABLE OF FIGURES Figure 1.1 Cut-off wall method Figure 2.1 Underground transition vectors of soil surrounding tunnel Figure 2.2 Volume lost components along the shield Figure 2.3 Affected area assessed by the displacement of the ground 11 Figure 2.4 Permeation grouting in tunneling 12 Figure 2.5 Jet-grouting in tunneling 13 Figure 2.6 Compensation in tunneling 15 Figure 2.7 Compaction method in tunneling .15 Figure 2.8 Micropiles method in tunneling 16 Figure 2.9 Principle of cut-off wall 17 Figure 3.1 Study methodology .18 Figure 3.2 Combined method is the Cut-off wall by Jet-grouting .19 Figure 3.3 Three typical methods of Jet-grouting method 19 Figure 3.4 Settlement trough is changed in both shape and depth 20 Figure 3.5 Verify the results of the reference experiment (without using the wall) with the theoretical result (Gaussian equation) 21 Figure 3.6 Horizontal displacement and settlement of cut-off wall used in cases of different variable parameters 21 Figure 3.7 Results of analysis by using FEM method to calculate cases of different types of wall system .23 Figure 3.8 Effective evaluation of cut-off wall system application .23 Figure 4.1 Research location of tunnels 26 Figure 4.2 General section at the location 26 Figure 4.3 Location of U-150 bore hole in project .27 Figure 4.4 Soil layers at the location 28 Figure 4.5 Settlement trough as Gaussian distribution curve 29 Figure 4.6 The model is simulated using Plaxis 2D software 31 Figure 4.7 Compare the results of the two calculation methods 32 Figure 4.8 Model simulates the reinforced wall system with Plaxis 2D .34 Figure 4.9 Settlement change according to modulus values of wall system 35 Figure 4.10 Relationship between surface settlement and modulus of wall system 36 Figure 4.11 Relationship between surface settlement and thickness of wall system 37 Figure 4.12 The relationship between two parameter and effective range 38 Figure 4.13 The efficiency of the HCM application is in the effective range 40 ii CHAPTER INTRODUCTION 1.1 General background In Vietnam, major cities are on the way of developing and growing rapidly in all fields from economic to social These also induce an increase in the demand of infrastructure system, especially the transportation system in urbans With experience from developed countries, Mass Rapid Transport (MRT) system is considered as the key to solve current congestion problems and enhance future development The most popular and prominent system in MRT is the subway system In recent years, the urban railway lines in the metro network in the two major cities of Hanoi and Ho Chi Minh city have been specifically planned and gradually implemented since 2006 In fact, with the land usage restricted in large cities, parts of railway network have been designed in underground construction, also known as railroad tunnels This type of construction requires many specialized construction equipment Currently, Tunnel Boring Machine or tunnel drilling machine using protection shield has been a specialized construction equipment widely used around the world With requirements of keeping the stability of structures on the ground, this construction method is highly appreciated in minimizing the impact of soil displacements when tunneling The scope of application can be mentioned as complicated geotechnical conditions, changing hydrology, unstable soft or rocky soil, construction with variable elevation Tunneling projects all over the world shows a high efficiency of TBM in underground construction However, there have been some cases of undesirable risks of ground displacement during the construction process or after a period of existence of underground structures in the ground These cause subsidence, which greatly effect on not only the above structure but also along area along the tunnel axis With many experiences from other countries, shallowed tunnels, which are placed not too deep, lead to more potential settlement caused serious impacts In addition, in Vietnam, TBM construction technology is still complicated and not familiar to domestic engineers Most of particular design and operation are in charge of foreign engineers Therefore, it is necessary to have more plenty studies from Vietnamese scientists and engineers in order to be capable of designing and producing technically reasonable criteria for the TBM technology 1.2 Problem statement The protection of existing structures (buildings) surrounding the tunnel is a necessary issue that have to be addressed in the tunneling design and construction The Cut-off wall method using the diaphragm wall system injected into the soil is one of the most effective methods in reducing the movement of the ground particles during tunnel construction using the TBM This method involves installing a physical diaphragm wall system with very low permeability or barrier walls around TBM excavation area By that installation, it prevents the change of groundwater seepage as well as displacements of soil mass when the ground is excavated, disturbed and lost (due to several reasons) In the world, this method can be a rigid wall system made of shaped steel (smooth wall) or a pour-in-place wall system (rough wall) which depend on each condition of geology and construction In Ho Chi Minh City, the urban railway No was tunneled close to the Opera House which is one of the most potential dangerous locations, thus the application of the Cut-off wall method (constructed by Jet-grouting technology to form the soilcement piles system) has brought a success in protecting this important historic building Regarding to the separation between tunnel and existing building by the Cut-off wall system, the development and expansion of the settlement trough above the tunnel is mitigated significantly Moreover, volume of settlement trough is also minimized as illustrated in Figure 1.1 The effectiveness of this separation depends not only on geometrical features such as relative dimensions, layout depth, and wall thickness but also on the hardness of fabricated materials and interactions with the surrounding soil Figure 1.1 Cut-off wall method It can be seen that the Cut-off wall method has a good effectiveness on strengthening and maintaining the stability of the ground at important construction locations affected by railway tunneling This thesis focuses on researching and analyzing the parameters that determine the efficiency of this method (geometrical parameters, material characteristics, wall-soil interaction) Combining finite element simulation and experimental results and observed data analysis will induce an effective adjustment for the settlement trough equation for this protection method 1.3 Aims of the study The tunnel safety assurance when tunneling design and construction is an essential issue not only on the safety of the tunneling process but also on the stability of existing nearby buildings Solving this problem faces to many relevant technical and technological challenges As discussed above, the cut-off wall system is one of the effective protection methods which has been applied successfully in Vietnam and all over the world With the aim of contributing to develop the urban railway system in Vietnam in future, the thesis will focus on the Cut-off wall system which will be applied for ground improvement and building protection when tunneling in soft soils Figure 4.7 Compare the results of the two calculation methods From the results obtained from the two calculation methods, the results of the two methods are relatively similar The semi-empirical method gives the largest settlement at the larger tunnel axis and the width of the settlement trough is relatively smaller than that of the finite element method It can be explained that the semi-empirical method does not accurately take into account such parameters as: soil characteristics (using the conversion coefficient K); tunnel structure parameters; impact between two tunnels constructed close to each other Therefore, in tunnel construction design, the semi-experienced method (notably the method of New & O'Reilly) is usually only used to forecast settlement before entering into detailed design by numerical method which is simulated using Plaxis 2D software With the problem of focusing on analyzing the largest settlement caused by tunnel construction, it is reasonable to deploy the analysis using the above model by the finite element method for the next steps The analysis process and results are presented in the next sections 4.3 Research on the solution of the wall system built by Jet-grouting Calculation of surface settlement when the diaphragm wall system has been constructed by jet-grouting technology is based on the theory of settlement by 32 layering method, with a part of the soil volume around the tunnel area enhanced with basic properties treated with high pressure grout Parameters of elastic modulus, specific gravity, friction angle and change of adhesive force will be changed compared to the original soil At that time, the problem becomes complicated and will take a long time, so Plaxis 2D software based on finite element method is chosen to be used to solve This problem is used to evaluate the settlement reduction effect of the diaphragm wall system that has reached sufficient strength before tunnel construction 4.3.1 Input parameters Parameters of the diaphragm wall system after constructed by jet-grouting technology to reinforce the ground are shown in below table Table 4.5 Design input parameter of cut-off wall system Parameter Value Unit Thickness 1st Variable m Unsaturated specific gravity (γunsat) 20 kN/m3 Saturated specific gravity (γsat) 22 kN/m3 2nd Variable kN/m2 Elastic Modulus (E) Poisson Ratio 0.2 Cohesion (c) 100 Friction angle (φ) 30 degree Expansion (ψ) degree Horizontal permeability coefficient (Kx) 0.5 m/day Vertical permeability coefficient (Ky) 0.5 m/day The model calculated using Plaxis 2D program with spray mortar treatment 33 is shown in the following figure Figure 4.8 Model simulates the reinforced wall system with Plaxis 2D In which the distance from the center of the tunnel to the inner wall is maintained at 5m, the thickness and elastic modulus of the wall system are changed The details of the change are as follows: - Thickness varies from: 0.5 m; 1.0 m; 1.5 m; 2.0 m; 2.5 m; 3.0 m; 3.5 m - Elastic modulus changed as follows: 10 MPa, 30 MPa, 50 MPa, 100 MPa; 300 MPa; 500 MPa, 750 MPa, 1000 MPa; 1500 MPa, 2000 MPa 4.3.2 Result of calculation 4.3.2.1 Relationship E-S with δ value fixed Initial calculation with values of modulus of variation from 100 MPa to 5000 MPa to investigate the effect of limiting settlement according to elastic modulus with a fixed value of δ of 1.5m (1.5m is common thickness value of walls constructed by jet-grouting) The calculation results are shown below 34 Figure 4.9 Settlement change according to modulus values of wall system 35 Figure 4.10 Relationship between surface settlement and modulus of wall system From the above analysis results indicate that: (1) When the value of elastic modulus (E) of the wall system increases to 50 MPa, the settlement at the survey site tends to increase This phenomenon can be explained because the original soil with specific gravity of 19.5 kN/m3 is replaced by soil mixed with spray mortar with γ = 22 kN/m3, which increases the self-weight of the subsoil, while the modulus of the wall system is not enough to prevent soil movement due to loss of tunneling volume (2) When the value of modulus E of the wall system increases from 70 MPa to 750 MPa, the surface settlement tends to decrease rapidly From 11.2 mm down to 6.7 mm for surface settlement (3) When the value of modulus E of the wall system increases from 750 MPa to 2000 MPa, the settlement also decreases gradually, but at a slow rate until it is negligible This causes construction difficulties in creating high strength mortar mixes, so it should be calculated in the design stage 4.3.2.2 Relationship δ -S with E value fixed The cut-off wall system in the ground, when considered per unit length, plays as a single pile that is subjected to lateral loads due to volume loss during impact tunneling Therefore, the hypothesis of the wall system as beams on a horizontal elastic foundation can be applied to this analysis From that, it can be seen that, when increasing the thickness of the wall system (height in the horizontal direction), the value of the modulus of resistance to the wall system also increases, making the horizontal deformation of the wall smaller The settlement of the soil behind the wall system will theoretically be improved In simulation analysis, the wall system is fixed with elastic modulus E = 100 MPa, which is the modulus value commonly used in the case of sandy soil The diaphragm wall system is studied to change the thickness of spray mortar in the range of 0.5m to 3.5m to investigate the effectiveness of settlement restriction The calculation results are shown in the graph below 36 From the analysis results from the graph, it shows that the relationship between wall thickness and settlement when the elastic modulus E = 100 MPa is fixed, the settlement reduction efficiency is almost linear, close to the original hypothesis Figure 4.11 Relationship between surface settlement and thickness of wall system Since then, the value of δ is used as a reference in the relationship of ground settlement along with the elastic modulus E Besides, the value of wall thickness constructed by jet-grouting technology is selected in about 0.5m to 3.0 depending on common construction equipment and machinery 4.3.2.3 Relationship E-S of surface settlement In the scope of this study, the value of elastic modulus E varies in the range from 10 MPa to 2000 MPa in relation to the change of layer thickness of the mortar reinforced wall system with the settlement of the ground surface at the survey site The values are composite and are represented as curves in the chart below 37 Figure 4.12 The relationship between two parameter and effective range From the chart above, with different thicknesses of the design wall system, the level of settlement reduction efficiency is appropriate when the elastic modulus is from 100 MPa to 300 MPa, corresponding to 10mm smaller than the allowable settlement Then the effect gradually decreased until about 1000 MPa and continued to decrease until negligible Therefore, depending on construction and geological conditions, priority will be given to technical parameters to achieve the appropriate thickness or modulus 4.3.3 Verify the effectiveness with site location monitoring data The subway tunnels run along a corridor in the central urban area of Ho Chi Minh City from a station at the Opera House to a station at the Ba Son shipyard and cut & cover area Within this corridor, the project may impact nearby and adjacent structures and buried utilities from tunneling and excavation works There are methods of protection were used and approached along the alignment Each method has its possible effects on buildings These methods shall be determined based on level of significance or level of risk for buildings and structures which is affected by underground construction 38 From the objective of study, the sections reinforced by cut off wall method constructed by jet-grouting will be compared with results from analysis model Detail of monitoring data is shown as Table 4.6 Table 4.6 Monitoring settlement data at reinforced locations *HCM CITY urban railway construction project (LINE 1) Monitoring Settlement Allowable Settlement Location Evaluation Maximum Different Maximum Different Operahouse (JET) 6.30 1/7353 10.00 1/1000 OK km 0+861 (JET) 11.10 1/3279 10.00 1/1000 OK km 0+775 19.20 1.4/1000 20.00 2/1000 OK km 0+755 19.70 0.88/1000 20.00 2/1000 OK km 0+735 16.80 0.42/1000 20.00 2/1000 OK km 0+695 12.70 - 20.00 2/1000 OK km 0+625 19.30 1.72/1000 20.00 2/1000 OK km 0+615 19.80 1.76/1000 20.00 2/1000 OK With a proposal that in order to cover all potential dangers when excavation by TBM through the important buildings, the engineers of the project had selected protective solution with high safety factor According that, cut-off wall systems around HCM Opera house were applied the structure of high ratio in both thickness and elastic modulus value These values are in the effective ranges which is calculated in this study (Figure 4.13) *HCM CITY urban railway construction project (LINE 1) 39 Figure 4.13 The efficiency of the HCM application is in the effective range 40 CHAPTER CONCLUSION & DISCUSSION 5.1 Conclusion The author's report studies the use of the cut-off wall system constructed by jet-grouting technology located in the ground to limit the impact of double tunnel construction in the geological area of Ho Chi Minh City The elements of the diaphragm wall system considered in the study include the deformation modulus and the thickness of the wall system By numerical method, through analysis of the finite element model by Plaxis 2D software, it has shown the close relationship between the elastic modulus and the thickness of the wall system When using a wall system with a thickness of less than 1.5 m, a spray mortar system with a small capacity must be used to form a module suitable for limited settlement Using high strength mortar will be difficult to spray into the soil In addition, when using a wall system with a thickness of more than 3.0m, the use of machines with a larger capacity faces many difficulties in the condition of limited space and narrow construction area in the city With the use of a wall system with an elastic modulus between 180 MPa and 250 MPa and a thickness between 1.5 m and 3.0 m, the settlement of the ground surface is within the allowable limit, ensuring the bearing capacity 5.2 Discussion The author's research has provided suggestions on two parameters of the diaphragm wall system, elastic modulus and thickness, in the problem of reducing settlement effects due to tunnel construction However, in practice, in order to study more accurately the settlement reduction effect, it is necessary to pay more attention to other parameters, such as: interaction between the ground soil and the pile system (roughness of the wall system in the soil), distance from the tunnel, deformation of the wall system to the deformation of the ground behind These factors will be studied and exploited by the author in the following studies Because of the short research time and the author's experience with 41 underground works (here is a railway tunnel constructed by TBM technology) is not much, the results may be not proper in any process stage of analyzing I am kindly looking forward to receiving 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International conference on innovations for sustainable and responsible mining, 250-269 46 ... can be seen that the Cut- off wall method has a good effectiveness on strengthening and maintaining the stability of the ground at important construction locations affected by railway tunneling. .. with finite element analysis models, together with analytical equations, the relationship between the input parameters and the impact of the cut- off wall system on the change of ground displacement. .. distribution, wall- soil interaction and soil parameters);  To simulate, verify, adjust the correlation among parameters to the effectiveness of the system;  To analyze and produce the equivalent equations